The Environmental Protection Agency (EPA) recently published the Mercury and Air Toxics Rule (MATS Rule) that will require all electricity generating units (EGUs) that burn fossil fuels to reduce mercury emissions levels within the next three years. Many of these units currently use or will use wet flue gas desulfurizers (wFGDs) to meet acid gas or SOx emission limits. A wFGD contacts combustion gas with an aqueous alkaline solution, which solution may be composed of magnesium compounds, sodium compounds, and slurries of lime or limestone to capture and neutralize acid gases, such as sulfur dioxide. The aqueous alkaline solution is commonly referred to as “wFGD liquor” or “scrubber liquor.” In a forced oxidation system, oxygen may be introduced into the wFGD liquor to oxidize sulfite to sulfate. In many cases, this forms gypsum (calcium sulfate), as the final byproduct of scrubbing. Other systems may utilize inhibited or natural oxidation scrubbing which results in sulfite salts or mixed sulfite/sulfate salts as byproduct.
Mercury entering EGUs as a contaminant of the fuel is released during combustion. Combustion gases exiting the boiler may contain mercury in three forms: particulate, oxidized, and elemental. Particulate mercury can be captured by particulate control devices such as electrostatic precipitators (ESPs) and fabric filters (FF). Oxidized mercury is water-soluble and as such wFGDs can absorb the oxidized mercury from the combustion gas into the liquid phase. Elemental mercury, which is insoluble in water, is difficult to capture using existing air quality control devices. Consequently, mechanical methods such as fixed bed catalysts (e.g., SCRs), and chemical additives (e.g., calcium bromide, hydrogen bromide, ammonium chloride) have been developed that oxidize elemental mercury in the gas phase for subsequent capture with a wFGD. The captured mercury leaves the process via wFGD blow down.
As oxidized mercury is water soluble, wFGDs are theoretically capable of capturing nearly 100% of the oxidized mercury in a combustion gas. However, data collected by the Department of Energy (DOE) as well as numerous laboratory and commercial studies have shown lower capture efficiencies. The lower efficiencies are the result of reduction of oxidized mercury to elemental mercury (e.g., Hg2+ to)Hg0 within the wFGD scrubber liquor. For example, one reduction reaction involves the oxidation of sulfite by ionic mercury in the wFGD to provide sulfate and elemental mercury. The result is an increase across the wFGD of elemental mercury content in the scrubbed combustion gas, and thus a decrease in total mercury capture as measured from fossil fuel to stack. This reduction of oxidized mercury in the scrubber and subsequent release is known in the industry as mercury re-emission. The loss in wFGD mercury capture efficiency due to mercury re-emission will prevent some EGUs from meeting the MATS Rule, necessitating installation of additional capital equipment.
Mercury re-emission is currently addressed with addition of sulfur-based additives, both organic and inorganic, or sulfur-based modified inorganics to chelate ionic mercury in the scrubber liquor, or through addition of absorbents such as activated carbon. In all these cases, the additive is introduced into the scrubber at an excess rate that has previously been shown to reduce re-emission. However, in none of these cases, have there been any development of control methodology that automatically controls the rate of addition of these re-emission control additives relative to normal operation. Variation in mercury concentration found in the fossil fuel, and therefore the combustion gas to be treated, results in periods of over and underfeeding of additive. Thus, in certain instances the lack of direct control results in overfeeding of the additives, which in turn leads to increased operating costs and higher waste generation.
There is a need in the art for methods of controlling the rate of addition of mercury re-emission control additives to provide the appropriate amount of additive to a scrubber without over- or underfeeding of additive.